What purpose do dead coils in the middle of a spring serve? For example, this spring from a ballpoint pen has two dead coils at each end and two in the center of the spring. Why add dead coils in the middle? What advantage does this create, or what disadvantage does this overcome? Is it a stability/buckling thing? Is it related to fatigue life? I've never seen this before and I couldn't find any other examples with a pretty extensive Google search, so I'm wondering if anyone has any insights. It's driving me bonkers as I just need to know. Thanks!
The dead coils on the ends of the spring furnish an (almost) flat surface, perpendicular to the spring axis, for the spring to engage the mating parts with. The dead coils in the middle effectively break the spring into two smaller springs, which will both compress in length upon loading without buckling, as NMech points out.
IMHO, It provides a stiffener midway that does somewhat arrest the rotational capacity during the buckling. I.e. if it where not there the mid section would be able to rotate more, and therefore accelerate buckling (that would result in the spring hitting the walls and probably jamming).